Gen

The term "Gen" probably arose from the term "general" as a rating used to indicate that a fan work was suitable for all audiences. It has evolved to refer to Fanfiction that doesn't focus on romantic and/or sexual relationships.

Defining exactly what is and isn't "Gen" however, can be difficult. Gen fics tend to be those that, like the show itself, have little sex or passing romantic relationships, but don't have them as central to the narrative. To what extent they can occur in a Gen fic can be contentious. Canonical relationships may be allowed if they are not the focus of the story e.g. mention of Dean and Lisa may be allowed but a story about the year Dean lived with Lisa would probably be disallowed. Wincest and Destiel are disallowed altogether.

Here we’re defining Gen as no romantic pairings. There can be background relationships or sexual content – e.g. the boys can be on a case helping a sweet old married couple with a poltergeist in their attic or trying to rid a BDSM club of a vengeful spirit. However, the boys, or whoever your main characters may be, should be single and we shouldn't be seeing a NC-17 blow by blows of the action in the club...or the nice old couple's bedroom. Let's just draw the curtain on that one.

In the early years of fandom Gen fic made up about 38% of Supernatural fic (Supernatural Fic Link Archive: Source). While figures are difficult to find, anecdotal evidence indicates this may be much higher than in other fandoms (For example see discussion on Gen as a minority genre in this post). This may be partly attributable to the proportion of writers and readers uncomfortable with Wincest or RPS when the fandom began, which made up the majority of the other fic in the fandom (about 39%). While some writers did only write Gen] fic, there were many in the fandom who will write stories that includes Gen, Slash and Het.

Hard or Dark Gen

"Gen" no longer relates to the rating of a fic. Gen fic may range from G through to NC-17. NC-17 fics tend to be those with high levels of violence, or horror themes. These are sometimes referred to as dark or hard Gen.

Brothers

brOTP is a portmanteau of bromance and "One true pairing". It is used across many fandoms to denote male characters that fans love seeing in an intensely close platonic relationship. In Supernatural fandom, the "bro" prefix also literally means brothers. It is used to refer to Gen fics about the brothers close relationship.

Gencest is the occasionally used term for a sub category of Gen stories. A mashup of Gen and Wincest, it describes stories where there is no sexual or romantic relationship between Dean and Sam, but where the story will focus on their intense emotional bond.

The term arose around 2009 (possibly from this post on LiveJournal) within Supernatural fandom, and was used to distinguish this sub-category of Gen stories from those where the focus was not on Sam and Dean's emotional relationship, but on a monster hunt, or the mythology or individual character study. At the time, Wincest stories were the main point of reference for stories focussing on the emotional connection between Sam and Dean, and yet many readers did not want to read stories featuring Wincest.

Terms such as Gencest also worked to attract readers who would generally only read Wincest. Some writers would note on their fic that there is "Wincest if you squint" or label it as "pre-slash" indicating that even if a sexual relationship wasn't explicit in a story, a reader could easily interpret it as if there was, or was going to be. Some writers would even produce two versions of their fic - a Gen version, and a Wincest one.

Even on the Show there have been references which blur the line - such as Zachariah's comment to Adam in 5.18 Point of No Return: You know Sam and Dean Winchester are psychotically, irrationally, erotically codependent on each other, right?

Profound Bond

BrOTP is rarely used for Gen stories about Dean and Castiel. Genstiel is another very rarely used term. Gen stories that focus on their emotional connection may be labelled as "DeanCas" (ratehr than Dean/Cas) or "Profound Bond" but these term are also used in Destiel fics.

RPF

Gen fics are exceedingly rare in RPF stories. Occasionally "pre-slash" will be used especially in non AU stories.